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How To Select an Inductor

Sanket Gupta pointed us to his latest article, on how to select an inductor and discuss about the various types of them.

Inductors are two-terminal components used for filtering, timing and power electronics applications. They store energy in the form of magnetic fields as long as a current is flowing. Inductors oppose a change in current by inducing an electromotive force (or e.m.f) according to Lenz’s Law. The inductor can be approximated as an open circuit for AC signals and as a short circuit for DC signals. The unit of inductance is Henry (H) .

There are four main factors that affect the amount of inductance of an inductor: the number of turns in the coil, coil area,coil length and core material. When inductors have a magnetic core made up of a ferromagnetic material it results in higher inductance. However, inductors with a magnetic core have losses such as hysteresis and eddy currents.

There are several parameters that are important to keep in mind while selecting an inductor:

Q factor, or Quality Factor, refers to the ratio of an inductor’s reactance to effective resistance. This value is frequency dependent and test frequency is often specified in datasheets. Q factor impacts the sharpness of the center frequency in an LC circuit. Usually, a high value of Q factor is preferred.

Self Resonant Frequency (SRF)describes the frequency at which an inductor stops working as an inductor. For RF circuits, self resonant frequency should be chosen such that it exceeds the operating frequency of the circuit. As the inductance is zero at the self resonant frequency, the Q factor is zero.

Saturation Current refers to the DC current which causes the inductance to drop by a specified value. The inductance drops because the core can only store a certain amount of magnetic flux density. While saturation current is related to the magnetic properties of the inductor, rated current is related to physical properties, and it describes the maximum DC current that can be passed in an inductor.

DC Resistance (DCR) refers to the resistance inherent in the metal conductor of the inductor, and it can be modeled as a resistor in series with the inductor. DC resistance is an important parameter in DC-DC converters design as the resistance leads to I^{2}R losses thus reducing the efficiency.

Tolerance is the variation in the inductance value of an actual inductor compared to the specified value in the data sheet. This could result in unwanted shift in frequency selection of an RF filter.

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